This invention relates to color developing materials used in carbonless copying systems and to receptor sheets and carbonless copying system comprising a substrate bearing said materials in a coating composition.
Copy systems employing chromogenic materials are well known. Such systems usually comprise microcapsules that contain a colorless chromogen (i.e., leuco dye) dissolved in a solvent. The microcapsules form a coating on the back or underside of a sheet of paper called a "transfer sheet" or CB (coated back) sheet. The transfer sheet is superimposed over a "receptor sheet" or CF (coated front) sheet, having a color developer for the chromogenic compound coated onto the front thereof. When the microcapsules containing the chromogen(s) are subjected to localized pressure, e.g., typewriter, ballpoint pen, or thelike, they are ruptured and the chromogenic material is released and transferred onto the underlying receptor sheet where it reacts with the color developer. The color developer is an electron acceptor substance such as an acid activated clay, or a low molecular weight phenol-formaldehyde resin. Such pressure sensitive copying system may include additional sheets interposed between the top sheet (CB) and the bottom sheet (CF). The interposed sheets are coated on their backside with chromogencontaining microcapsules and on their front side with a color developer. These sheets are known on CFB (coated front and back) sheets. As used herein, the term "transfer sheet" includes any substrate bearing a coating of electron accepting material and includes CF and CFB sheets as previously described.
Chromogenic compounds comprising colorless dye intermediates are conventional. Exemplary of the colorless dye intermediates which are contemplated for use in this invention are leuco dyes such as crystal violate lactone (CVL), derivatives of bis(p-dialkylaminoaryl) methane, dilactones, ureido fluorans, and bisfluorans such as disclosed in U.S. Pat. Nos. 2,981,733, 2,981,738, 3,819,396, and 3,821,010. These dye intermediates are colorless in a neutral or alkaline medium and react to form a visible color in an acidic medium. Thus, when a capsule containing such a compound is ruptured and the compound is discharged onto an absorbent, acidic, electron-acceptor material, such as a paper web coated with an organic or inorganic acid material, a visible color appears on the absorbent material at the point of contact.
Heretofore, pressure-sensitive copy systems have employed acidic clays, and more recently, oil soluble phenolic resins and/or their metal salts as the receptor materials as disclosed in U.S. Pat. Nos. 3,672,935, 3,723,156 and 3,427,180. Receptor sheets employing acidic clays and phenolic resins as the electron acceptor substances have major disadvantages. For example, images formed on acidic clays are succeptible to deterioration due to heat, moisture and light upon prolonged exposure to atmospheric conditions. Furthermore, acidic clays present severe rheological problems such as extremely high viscosities and dewatering during the preparation of the coating formulation and the application of said coating formulation to the paper web. Additionally, papers coated with acidic clays are highly abrasive and have a tendency to yellow severely upon aging. The oil-soluble phenolic resins offer some improvement over the acidic clays such as improved resistance to moisture, but they too have major drawbacks. For example, prolonged exposure of receptor coatings containing phenolic resins to heat and/or light causes the "splitting off" of phenolic groups and results in an overall degradation of the resin. Such degradation of the resin is reflected in yellowing of the coated sheets, fading of the formed image, and loss of image-forming ability of the receptor sheet. Furthermore, the presence of such free phenolic groups present environmental and health hazards.
The use of certain aromatic carboxylic acids as electron acceptors or color developers in carbonless copying systems is also known. For example, U.S. Pat. Nos. 3,488,207, 3,871,900, 3,934,070, and 3,983,292 disclose the use of such aromatic carboxylic acids and/or their metallic salts as reactive materials for chromogens. These aromatic carboxylic acids are capable of developing images which are superior in intensity and stability to those formed by acidic clays and phenolic resins. Several of these aromatic carboxylic acids, however, present severe problems such as extremely high viscosities and excessive foaming during the preparation of the coating solution and the application of said solution to the web. These problems render the use of such materials impractical in large scale, commercial manufacturing operations. Furthermore, several of these aromatic carboxylic acids possess some undesirable features such as, slow rate of reaction with the chromogen, low sublimation point resulting in an unstable receptor sheet, and form images of low intensity and stability.